The database has been produced by BBC News from a report on immigration, published by the Institute for Public Policy Research (IPPR), a think tank.

The report is largely based on data from the UK census, a huge survey carried out every decade and last conducted in 2001.

One of the census questions asks people where they were born. All the answers were collated for the IPPR by an expert team within the geography department at Sheffield University. The answers were also applied to maps of Britain, so concentrations of different immigrant groups could be identified.

Most of Born Abroad consists of those collated figures - with some extra processing by the BBC to produce, for example, percentage change data - and the maps.

On top of that, the IPPR report used figures from the Labour Force Survey (LFS), which are in turn used in the economics section of Born Abroad. The LFS is a quarterly survey of 60,000 households that includes questions on personal details such as age and country of birth, and also collects data including earnings and employment status. Five years of Labour Force Surveys (2000-2004) were used by the IPPR to provide a large enough number of immigrants to analyse.

Unlike some other surveys, census and LFS data includes all foreign-born people regardless of how long they have been in the UK, how much longer they intend to stay in the UK or their immigration status.

Some people do not participate in the census, but the Office for National Statistics adjusts the figures to take that into account. Statisticians consider the census to be the most accurate source of information available on the size and composition of the UK population. It includes children.

Born Abroad is split into four sections that can be browsed via four separate tabs at the top of the page.

"Overview" is the introduction and contains figures and maps for Britain as a whole.

"Around Britain" gives an overview of the figures for England's nine regions plus Scotland and Wales, and provides links to individual pages for each, which contain figures at a much more detailed local level.

"Countries of Birth" contains the figures categorised according to where immigrants were born, and provides links to individual country pages where further detail is available.

"Economics" contains separately sourced data on the employment and wage levels of different immigrant groups.

Many of the tables in Born Abroad can be sorted by clicking on the column titles.

The figures in Born Abroad can be viewed for Britain as a whole and then for England's nine regions, plus Scotland and Wales (see map below).

On the individual pages for each region and Scotland and Wales, accessed from the "Around Britain" tab, figures are available for the numbers of people born outside the British Isles at more local level. These are based on a unique unit, which Sheffield University called a "tract" and Born Abroad calls a local area.

They were needed in order to provide a consistent geography over time. They do not exactly match any existing geographic units such as local council areas, parliamentary constituencies or electoral wards because their boundaries change over time as populations shift.

All 1,282 local areas have similar-sized populations and are roughly equivalent to half a parliamentary constituency. All have been labelled using easily recognisable names.

Two types of map are used in Born Abroad, standard (below left) and cartographic (below right), which are always shown alongside each other with the same information in them.

The cartographic maps are made up of small squares, each of which represents an area of roughly equal population size. As a result, densely populated areas - like London - take up much more space than sparsely populated areas, like the Scottish Highlands. The cartographic maps make it is easier to see the information being displayed for the densely populated areas.

To help understanding, you can choose whether to have Britain's cities labelled or not using the tickbox.

Separate figures for every country of birth are not available from the census. Where countries are not listed separately, they have been grouped together with others in the same region, and given appropriate titles.

These figures show what percentage of the total population in Britain, in a region or in a local area (depending on the table being viewed) consists of people born outside the British Isles.

For example, on the "Regions" tab, on the table titled "Distribution of people born abroad", the top row shows that in 1991 in the East Midlands 4.46% of the total population was born outside the British Isles, which went up to 5.38% in 2001, representing an increase of 0.92%.

On the "Around Britain" and "Countries of birth" tabs, it applies because the countries included in each census are not constant. For example, the figure for Afghanistan was not separately available from the 1991 census data but was in 2001.

On the economics tab "n/a" is used where the sample size for the country in question was too small for the response to be considered meaningful.

Because the data has been divided up at local level using a unique geographic unit (see above), which does not match the postcode database we use with other sets of statistics, such as election results.

These terms are used on the "Economics" tab, where the data is based on Labour Force Surveys conducted between 2000 and 2004.

New immigrants are those who arrived in the UK in the 15 years before the survey date. So, for the Labour Force Survey of 2004, new immigrants were those who arrived in 1990 or later, settled immigrants were those who arrived in the UK before 1990.

On the "Economics" tab, low earners are people earning less than half the UK median wage, which over the time of the Labour Force Surveys from which these figures were taken (2000 to 2004), was £149.20 a week.

High earners are people earning more than £750 a week, which is one of the UK's highest earning brackets.

The census figures that make up the bulk of Born Abroad only show the country people were born in, not their ethnicity or citizenship. For example, the children of British military personnel born in Germany will be included in the figures for Germany, even though they are likely to be British.

Also, the figures only reflect the picture up to 2001, since when some significant events have had an impact on immigration to Britain, including, for example, enlargement of the European Union and the war in Iraq.

The economic figures are more up-to-date, as they are based on Labour Force Surveys up to 2004.